Instrumentation is widely used to measure the level of a coolant, such as water, in the reactor vessel of a boiling water reactor or in the pressurizer or steam generator of a pressurized water reactor as well as of a common boiler. Although other coolants may be used, the most common coolant used in boilers and reactors is water. Therefore, the coolant will be referred to hereinafter as water.
The results of water level measurement are important data necessary for the water level control, feed-water control, and other safety measures for the boiler or reactor. Accordingly, the water level instrumentation measurement system must operate correctly within a given allowable error even in case of an accident, such as a leak or break in the coolant system boundary. The output of the water level instrumentation measurement system is supplied to various control machinery and equipment such as (in the case of nuclear reactors) the reactor protection system, the water level control machinery and equipment, and the water supply control system for maintaining the safety of the reactor. Therefore, the water level instrumentation measurement system must provide correct water level signals even in the case of a nuclear reactor accident.
The Water Level Instrumentation Measurement System
The instrumentation of the type mentioned above is conventionally known and is illustrated in FIG. 1 as applied to a pressurized water reactor or a boiling water reactor. The water level instrumentation measurement system 1 is attached to a pressure vessel 10. Pressure vessel 10 and most of water level instrumentation measurement system 1 are disposed in a drywell (or primary containment) 12 adjacent the reactor building 14. A portion of water level instrumentation measurement system 1, including a reference leg 16 and a differential pressure detector 18, is located in reactor building 14.
Sufficient cooling water 20 is supplied to pressure vessel 10 to provide a water level 22 covering the reactor fuel 24. Cooling water 20 is confined in the lower space 26 of pressure vessel 10 below water level 22. Fuel 24 is supported in pressure vessel 10 by a base 32 and heats cooling water 20. When the reactor is operated in a normal state, cooling water 20 in pressure vessel 10 is heated and high-temperature (about 550.degree. F.), high-pressure (about 1,000 psia) steam 30 is collected in the upper space 28 of pressure vessel 10 above water level 22. Steam 30 is transmitted to, for example, a steam turbine (not shown) through a main steam line 34.
Water level instrumentation measurement system 1 is attached to pressure vessel 10 through an upper tap 36, above water level 22, via a steam inlet 38. Steam inlet 38 is about 1 inch in diameter, upwardly inclined, and surrounded by insulation 40. Insulation 40 may be reflective metallic, asbestos, perlite, or the like. At least one condensing chamber 50 is connected to steam inlet 38. Reference leg 16 connects condensing chamber 50 to differential pressure detector 18. Water level instrumentation measurement system 1 returns to pressure vessel 10 through a lower tap 42, below water level 22, via a variable leg 44.
Water level instrumentation measurement system 1 shown in FIG. 1 operates by measuring the difference in pressure between two columns of water. Steam 30 from pressure vessel 10 is directed through upper tap 36 and steam inlet 38 to condensing chamber 50. Because the temperature of the chamber wall of condensing chamber 50 is lower than the temperature of introduced steam 30, steam 30 condenses on the chamber wall defining vapor space 48. Steam 30 condensing in condensing chamber 50 fills and then maintains a constant water height in reference leg 16. The temperature of water 20 a short distance from condensing chamber 50 in reference leg 16 is about 150.degree. F. It is extremely important for proper measurement of water level 22 that a constant height of water be maintained in reference leg 16. Moreover, the pressures inside both upper space 28 of pressure vessel 10 and vapor space 48 of condensing chamber 50 are maintained at the same level.
The water surface 46 inside condensing chamber 50 never rises above the bottom edge of the opening 52 of steam inlet 38 into condensing chamber 50. This is because the upward incline of steam inlet 38 allows steam inlet 38 to serve as a drain pipe. Excess condensate returns to pressure vessel 10 through steam inlet 38.
The elevation of condensing chamber 50 is higher than that of differential pressure detector 18 in reactor building 14. Variable leg 44 (through lower tap 42) connects to pressure vessel 10 at an elevation below the bottom of the pressure vessel water level range which water level instrumentation measurement system 1 will measure. The variable level water column height varies as pressure vessel water level changes; the reference leg water column height should remain constant. Differential pressure detector 18 in reactor building 14 measures the pressure difference between reference leg 16 and variable leg 44 and converts it (using the values of density of water and steam in pressure vessel 10) to a water level indication for use during plant operation.
Ideally, steam inlet 38 which connects condensing chamber 50 to pressure vessel 10 should have minimal energy loss. Insulation 40 is provided around steam inlet 38 for this purpose. Assuming minimal energy loss from steam inlet 38, most of the condensation of steam 30 will occur in condensing chamber 50.
Steam 30 produced in pressure vessel 10 contains trace amounts of non-condensable gases. In the nuclear industry, non-condensable gases are generated as part of the fission process and are primarily a stoichiometric mixture of hydrogen and oxygen (but include smaller amounts of other elements such as nitrogen). In other applications, the source of these non-condensable gases is air. Steam 30 from pressure vessel 10 carries the non-condensable gases with it to condensing chamber 50.
When steam 30 enters condensing chamber 50 and condenses, gases are not condensed and, hence, remain to raise the concentration of the non-condensable gases in vapor space 48 of condensing chamber 50. As the concentration of the non-condensable gases increases in condensing chamber 50, some of the gases will go into solution in water 20 in condensing chamber 50. The condensate (with its dissolved gases) will spill out of condensing chamber 50 and return to pressure vessel 10--maintaining a constant height of water in reference leg 16. The condensate returning from condensing chamber 50 to pressure vessel 10 carries dissolved, non-condensable gases back to pressure vessel 10. At equilibrium, with no leak in reference leg 16, the surplus condensate (spillover flow) will exit condensing chamber 50 through steam inlet 38 such that its mass flow rate will be equivalent to that entering from pressure vessel 10. Moreover, the accumulation or build-up of non-condensable gases in condensing chamber 50 will reach an equilibrium: the amount of non-condensable gases dissolved in the condensate returning to pressure vessel 10 from condensing chamber 50 equals the amount of non-condensable gases carried by steam 30 to condensing chamber 50.
Thus, the primary mechanism for removal of the non-condensable gases from condensing chamber 50 to pressure vessel 10 is via the dissolved gases contained in the spillover flow. When an ideal system is working properly, non-condensable gases should not accumulate in condensing chamber 50 beyond the equilibrium level. Under these ideal conditions, the amounts typically expected for oxygen and hydrogen in steam 30 are 13 ppmv and 25 ppmv (parts per million by volume), respectively. An equilibrium concentration of non-condensable gases is also present in the condensate returning to pressure vessel 10 via steam inlet 38. To arrive at this equilibrium, the equilibrium concentration of each non-condensable gas in condensing chamber 50 is based on Henry's Law.
B. The Problem
1. Non-Condensable Gas Accumulation
There are two primary ways in which non-condensable gases can accumulate in condensing chamber 50 in excess of the ideal equilibrium condition. First, a leak out of reference leg 16 may reduce spillover flow and limit the non-condensable gas removal. Such a leak condition could cause water surface 46 in condensing chamber 50 to decrease below the lower edge of opening 52, thereby eliminating the ability of the non-condensable gases to be removed from condensing chamber 50 via the spillover flow.
Second, the amount of gas that may be dissolved in water is directly proportional to the partial pressure of gas above the water surface, according to Henry's Law (which states that the concentration of dissolved gas in water is a product of the gas solubility in water, a parameter dependent upon temperature, and the partial pressure of the gas above the water's free surface). Gas is released or "stripped" from the return condensate flow (which is "degassed") because the partial pressure of non-condensable gases in steam inlet 38 is less than the concentration in condensing chamber 50; thus, the dissolved gas in the condensate return is not in equilibrium with the partial pressure of the gas in steam inlet 38. This stripping action provides a concentrating mechanism that returns non-condensable gases to vapor space 48 above reference leg 16. Stripping is accelerated in turbulent flows, as would be encountered in sloped pipe segments, and at nucleation sites on the pipe walls, which would increase in number with increases in pipe length.
In short, a degassing of the condensate (spillover) flow which is returning to pressure vessel 10 may release non-condensable gasses. The rate of degassing increases with the length of steam inlet 38 and with discontinuities in the spillover flow caused by pipe bends, varying pipe cross-sectional area, burrs, gaps, or roughness on the inside of steam inlet 38. These degassed non-condensables will be returned to condensing chamber 50 with the upward flow of steam 30 from pressure vessel 10.
Through one or both of these mechanisms (or other mechanisms not yet known), therefore, non-condensable gases may collect in excess of the ideal condition in condensing chamber 50 at the top of reference leg 16.
2. Migration Into Reference Leg
As steam condensate flows down the walls of condensing chamber 50 and into reference leg 16 to maintain the reference leg liquid inventory, there is a potential for non-condensable gases to dissolve into water 20 in condensing chamber 50 and to migrate into reference leg 16. This dissolved gas solution could be carried into reference leg 16 via diffusion, thermal convection, a leak in reference leg 16, or a combination thereof. The fastest means of conveying any high-concentration solution into reference leg 16 would be a leak. Very small water leaks (less than 0.1 lbm/hr) are all that are needed to bring reference leg 16 to an equilibrium level of dissolved, non-condensable gases in reference leg 16. Over time, non-condensable gases may accumulate in reference leg 16 of water level instrumentation measurement system 1.
3. Release Upon Depressurization
During depressurization, gas comes out of solution and, eventually, may form a bubble. The non-condensable gases dissolved in water 20 in reference leg 16 may come out of solution as reference leg 16 depressurizes. Depressurization may occur slowly under normal operating conditions (such as shutdown) or rapidly under transient or emergency conditions. During a large break loss of coolant accident (LOCA), for example, non-condensable gases in reference leg 16 could come rapidly out of solution. In either case, the non-condensable gases expand and can displace much of the water contained in reference leg 16.
In summary, all three of the following events must occur to cause such displacement: (1) an elevated level of non-condensable gases must exist in condensing chamber 50; (2) the gases must be drawn into reference leg 16 by, for example, a small leak in a fitting, valve, manifold, or the like; and (3) depressurization must occur.
It has been demonstrated that non-condensable gases in solution in reference leg 16 can cause significant errors in measuring water level 22 during rapid depressurization and normal shutdowns (slow depressurizatons). This error is due to displacement of water 20 in reference leg 16 as the non-condensable gases come out of solution and expand as the pressure drops. The amount of water 20 lost from reference leg 16 depends upon the geometry of reference leg 16, the amount and composition of initial non-condensable gases, and the depressurization rate. Water loss in reference leg 16 directly impacts the reference pressure sensed by differential pressure detector 18 and, consequently, the measurement of water level 22 in pressure vessel 10. The result is a non-conservative (high) measurement.
The concern with errors in measuring water level 22 during normal depressurization was confirmed during 1992. An operating boiling water reactor plant experienced an automatic reactor trip and was implementing a plant cooldown when operators observed momentary increases in indicated water level ("notching"). After the coolant system depressurized to significantly below the coolant setpoint of 98 psig, indicated water level became erratic and a 32-inch error developed. An extended period elapsed before the error was recovered.
Incorrect measurements of water level 22 during normal operations present one set of problems. Because operators use such measurements when implementing emergency operating procedures during abnormal conditions to determine whether adequate core cooling is achieved, however, an erroneous high indication of water level 22 would result in ambiguous or misleading information to the operators during a dynamic depressurization event. This creates another, potentially catastrophic, set of problems.
Clearly, some modification of water level instrumentation measurement system 1 is required to prevent non-condensable gases from accumulating in reference leg 16. The priority for this modification is high because intrusion of non-condensable gases into reference leg 16 can render water level indication indeterminate and inaccurate. The Nuclear Regulatory Commission (NRC), in NRC Bulletin 93-03 dated May 28, 1993, required all boiling water reactor licenses to implement a hardware modification to their water level instrumentation measurement systems "to ensure the level instrumentation system design is of high functional reliability for long-term operation. This includes level instrumentation performance during and after transient and accident scenarios initiated from both high pressure and reduced pressure conditions." The modification was required to be implemented before reactor start up following any shut down to a "cold shut down" condition after Jul. 31, 1993.
C. Attempted Solutions
The boiling water reactor industry embarked on a program in July 1992 to address and resolve the identified concern with reactor water level instrumentation measurement system 1. To solve the problem, one of the three causes (identified above) must be removed. Depressurization must and will occur. It is difficult to prevent the minor leaks that can draw non-condensable gases into reference leg 16. Efforts have focused, therefore, on removing non-condensable gases from condensing chamber 50 before elevated levels can accumulate.
To provide a viable solution, the modification to water level instrumentation measurement system 1 should be transparent to the operator. It must not offset water level 22 during operation. Preferably, it will avoid active components and require neither maintenance nor operator activities to ensure proper operation. Finally, as modified, water level instrumentation measurement system 1 must be in service continuously.
A number of alternative modifications to water level instrumentation measurement system 1 have been proposed to solve the problem of measurement errors caused by non-condensable gases. One proposal tentatively selected by the industry (i.e., the Boiling Water Reactor Owners' Group) is to vent condensing chamber 50 to the main steam line. This modification uses positive steam flow through condensing chamber 50 to prevent non-condensable gases from accumulating in condensing chamber 50.
The main steam line is subject, however, to pressure fluctuations measured in many feet of water. These pressure fluctuations cause significant signal noise in the water level instrumentation, which generates a water level signal from a pressure differential measured in inches of water. Thus, the water level signal obtained when condensing chamber 50 is vented to the main steam line is both inaccurate and full of noise.
A second proposed modification is to add an accumulator below condensing chamber 50 to reduce the water level measurement error to a small, quantifiable value. The utility industry has rejected this proposal. Although it improves the operation by restricting the error to a known value, the proposed modification fails to address any of the causes of the error.
A third proposed modification is to manually backfill reference leg 16 with regular frequency. The utility industry rejected this proposal because it would require extensive maintenance support. Although the frequency of backfill which would be required is indeterminate, there is no doubt that the time between required backfill steps would be very short. Moreover, a certain level of operational risk would accompany each backfill operation.
An automatic keepfill system, providing continuous backfill of reference leg 16, is a fourth possible solution. The NRC Staff endorsed the continuous backfill modification as an acceptable solution to the degassing problem. Such a system is under investigation, and has been adopted by much of the utility industry, but requires relatively extensive modifications to piping and valving. Moreover, the keepfill system may not be feasible at certain plants.
A fifth proposed modification is to install a temperature monitor on condensing chamber 50 and backfill only when the temperature decreases. The utility industry has rejected this proposal. Decreasing temperature may needlessly require plant shutdown for containment entry. Moreover, the temperature instrumentation would require maintenance, penetrations of containment would be required to install cable, and operator interface would be required to monitor the temperature.
Sixth, it has been proposed that the condensing chamber be placed below the vessel nozzle. The utility industry has rejected this alternative because it would introduce a downward-sloped section of pipe (from the pressure vessel to the condensing chamber). Operators would not know whether that section of pipe was full of water.
Finally, the concept of venting condensing chamber 50 to variable leg 44 has been proposed. Such a modification would use steam flow through condensing chamber 50 to prevent non-condensable gases from accumulating in condensing chamber 50. The timing of responses required by NRC Bulletin 93-03 effectively discouraged utilities from pursuing this modification, especially because the NRC Staff stated that it would have a lot of questions regarding installation of such a modification. The utility industry has not pursued the concept of venting condensing chamber 50 to variable leg 44, electing instead to adopt an automatic keepfill system as accepted by the NRC.
To overcome the shortcomings of existing water level instrumentation measurement systems, a modified water level instrumentation measurement system is provided. An object of the present invention is to provide an improved system that measures the water level in a pressure vessel in a stable manner even in the case of a LOCA, depressurization event, or transient. A related object is to provide, as one component of the water level instrumentation measurement system, a condensing chamber which tolerates non-condensable gases present in the system. Another object is to prevent the accumulation of non-condensable gases in condensing chambers.